The Design of Rolling Bearing Mountings

98 Two-bearing screen with straight-line motionBasically, a two-bearing screen with straight-linemotion consists of two contra-rotating, synchronouscircular throw systems.Operating dataScreen box weight G = 33 kN; imbalance weight G 1 =7.5 kN; amplitude r = 0.008 m; speed n = 900 min –1 ;number of bearings z = 4 ; acceleration due to gravityg = 9.81 m/s 2 .Bearing dimensioningThe bearing loads of a linear motion screen vary twicebetween the maximum value F rmax and the minimumvalue F rmin during one revolution of the eccentricshafts.For calculation of these loads, the distance R betweenthe centres of gravity of imbalance weight and the pertinentbearing axes is required. Weights G and G 1 , amplitudeof linear vibration r and distance R have thefollowing relationship:G · r = G 1 · (R – r)In this example R = 0.043 mWhen the centrifugal forces act perpendicular to thedirection of vibration, the maximum radial load F rmaxis calculated as follows:F rmax = 1/z · G 1 / g · R · (π · n/30) 2 == 1/4 · 7.5 / 9.81 · 0.043 · (3.14 · 900/30) 2 = 73 kNThe radial load is at its minimum (F rmin ) when thedirections of centrifugal forces and vibration coincide.The radial load is thenF rmin = 1/4 · G 1 /g · (R - r) · (π · n/30) 2 == 1/4 · 7.5/9.81 · 0.035 · (3.14 · 900/30) 2 = 59.4 kNSince the radial load varies between the maximum andminimum according to a sinusoidal pattern, theequivalent dynamic load P with the supplementaryfactor f z = 1.2 is thus:P = 1.2 · (0.68 · F rmax + 0.32 · F rmin ) == 1.2 · (0.68 · 73 + 0.32 · 59.4) = 82.4 kNWith the index of dynamic stressing f L = 2.53 (L h =11,000 h) selected for vibrating screens and the speedfactor f n = 0.372 (n = 900 min –1 ) the required dynamicload ratingC = f L /f n · P = 2.53/0.372 · 82.4 = 560.4 kNThe spherical roller bearing FAG 22320E.T41A with adynamic load rating of 655 kN is chosen.Machining tolerancesThe locating bearings of the two eccentric shafts are atthe gear end, the floating bearings at the drive end. Theinner rings (point load ) are have loose fits, i. e. theshaft is machined to g6 or f6. The outer rings are circumferentiallyloaded and tightly fitted in the housingbore (P6).Lubrication, sealingOil lubrication. For lubricating the spherical rollerbearings at the locating end, the oil thrown off by thegear suffices. A flinger ring is provided for this purposeat the opposite end. Baffle plates (A) at the housingfaces maintain an oil level reaching about the centrepoint of the lowest rollers. The oil level is such that thelower gear and the flinger ring are partly submerged.The oil level can be checked with a sight glass.A flinger ring and a V-ring in the labyrinth providesealing at the drive shaft passage.FAG 140